Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition, overlay and other parameters of nanoscale structures.
In semiconductor device manufacturing, etch processes and deposition processes are critical steps to define a device pattern profile and layout on a semiconductor wafer. Thus, it is important to measure films and patterned structures to ensure the fidelity of the measured structures and their uniformity across the wafer. Furthermore, it is important to provide measurement results quickly to control the on-going process and to adjust settings to maintain required pattern or film uniformity across the wafer.
In most examples, precise monitoring of a semiconductor manufacturing process is performed by one or more stand-alone (SA) metrology systems. SA metrology systems usually provide the highest measurement performance. However, the wafer must be removed from the process tool for measurement. For processes undertaken in vacuum, this causes significant delay. As a result, SA metrology systems cannot provide fast measurement feedback to process tools, particularly process tools involving vacuum. In other examples, integrated metrology systems or sensors are often attached to process equipment to measure wafers after a process step is completed, but without removing the wafer from the process tool. In other examples, in-situ (IS) metrology systems or sensors are employed inside a processing chamber of a process tool. Furthermore, an IS metrology system monitors the wafer during the process (e.g., etch process, deposition process, etc.) and provides feedback to the process tool performing the fabrication step under measurement.
In one example, structures subject to a reactive ion etch process are monitored in-situ. In some fabrication steps, the etch process is required to etch completely through an exposed layer and then terminate before substantial etching of a lower layer occurs. Typically, these process steps are controlled by monitoring the spectral signature of the plasma present in the chamber using an emission spectroscopy technique. When the exposed layer is etched through and the etch process begins to react with a lower layer, a distinct change in the spectral signature of the plasma occurs. The change in spectral signature is measured by the emission spectroscopy technique, and the etch process is halted based on the measured change is spectral signature.
In other fabrication steps, the etch process is required to etch partially through an exposed layer to a specified etch depth, and terminate before etching completely through the exposed layer. This type of etch process is commonly referred to as a “blind etch”. Currently, the measurement of etch depth through partially etched layers is based on near-normal incidence spectral reflectometry.
In some examples, the wafer under measurement includes periodic patterns. These patterns exhibit unique reflectivity signatures that can be modeled. Thus, model based spectral reflectometry measurement techniques are suitable for estimating critical dimensions of patterned wafers. Unfortunately, currently available in-situ monitoring tools based on spectral reflectometry lack the precision required to meet future fabrication process requirements.
In some examples, scatterometry-based IM and IS systems are employed in the semiconductor industry. Exemplary IM and IS systems are described in U.S. Pat. No. 6,917,433 assigned to KLA-Tencor Corporation, the contents of which are incorporated herein by reference. An optical endpoint detection metrology system that compares a measured signal to a pre-determined “endpoint” signal or calibration curve is disclosed in U.S. Pat. No. 6,764,379, assigned to Nova Measuring Instruments, Inc., the contents of which are incorporated herein by reference. Although the aforementioned patent documents describe tools and methods for controlling etch and deposition processes, they are unable to determine process endpoints for complex patterns fabricated on modern semiconductor wafers with multiple pattern layouts under measurement. For example, current IS or IM systems fail to account for multiple process parameters that affect the measured signal. Although etch or deposition time is one process parameter impacting the measured device structure, there are other process parameters that affect the measured structure in a similar manner. The inability of prior art systems to account for multiple parameters affecting the measured signal limits their measurement effectiveness, particularly for complex patterned structures.
In many practical examples, the semiconductor wafer under measurement includes homogeneous regions of periodic patterns and also non-homogeneous regions including support circuitry, scribe lines, etc. For example, on a memory wafer, the typical size of the homogeneous region is about 50 microns square surrounded by a non-homogeneous region of a few microns surrounding the homogeneous region. Currently available in-situ monitoring tools illuminate the wafer with a collimated beam that illuminates a large circular area of the wafer, including homogeneous and non-homogeneous regions. Typical illumination spot sizes are ten millimeters in diameter, or larger. The reflected light collected over this large area is mixed and analyzed by a spectrometer. Mixing the reflectivity signals from homogeneous and non-homogeneous regions on the wafer fundamentally limits the performance of the metrology system (i.e., measurement accuracy is limited).
The problem of mixing of reflectivity signals from homogeneous and non-homogeneous regions of the wafer is difficult to solve optically because it is not possible to place illumination and collection optics near the wafer within the reactive plasma chamber. This limits the maximum achievable numerical aperture (NA) and the minimum achievable illumination spot size. Without the ability to optically focus on a small homogeneous region of the wafer with minimum spill-over onto the surrounding non-homogeneous region, current systems cannot overcome the limits to measurement accuracy due to mixing of reflected signals.
In summary, ongoing reductions in feature size impose difficult requirements on IM and IS metrology systems, particularly those integrated with etch and ion implant process tools. Optical metrology systems must meet high precision and accuracy requirements to enable adequate process control. Thus, improved metrology systems and methods are desired to measure films and patterned structures to ensure their fidelity and uniformity across the wafer.